1. Field of the Invention
The present invention relates to a tunable dispersion compensation device for dynamically compensating for chromatic dispersion in an optical fiber which is a transmission path for use in an optical fiber communication system, an optical receiver including such a tunable dispersion compensation device, and an optical fiber communication system including such a tunable dispersion compensation device.
2. Description of the Prior Art
In recent years, as it has become desirable to utilize many channels (i.e., many optical signals) over a wider range of wavelengths to carry a lot of information via an optical fiber which is a transmission path in an optical fiber communication system, such as a wavelength division multiplexing (WDM) system, chromatic dispersion (group delay dispersion) in the optical fiber has required more precise compensation. Chromatic dispersion in an optical fiber causes spectral components of different wavelengths included in an optical signal to propagate through the optical fiber at different speeds, thereby inducing pulse broadening in the optical signal. For example, a single mode fiber used for optical fiber communication systems provides abnormal dispersion (negative group velocity dispersion) for an optical signal of a wavelength of 1550 nm, the chromatic dispersion having a positive sign and being typically equal to about 17 ps/nm/km. In other words, spectral components of shorter wavelengths included in an optical signal propagate through the single mode fiber faster than other spectral components of longer wavelengths, and the pulse width of an optical signal having a spectral width of 1 nm increases only by about 17 ps every time the optical signal propagates through a 1 km length of the single mode fiber, for example. Two adjacent pulses in an optical pulse train that propagates through an optical fiber can thus overlap with each other at a high data rate. Such pulse overlapping can cause errors in data transmission.
In order to compensate for such chromatic dispersion in an optical fiber which is a transmission path, a dispersion compensation fiber and an optical waveguide, such as an optical fiber, including a chirped grating, which provide group velocity dispersion of a sign opposite to the dispersion in the optical fiber have been developed. On the other hand, there is a problem that chromatic dispersion in an optical fiber may vary with time because of a change in the temperature of the optical fiber, a change in the connection of the optical fiber, a change in the stress placed on the optical fiber due to external forces, and so on. Since those prior art dispersion compensation devices can only compensate for a fixed amount of chromatic dispersion, they cannot deal with such a problem. Particularly, in optical fiber communication systems that operate at 40 Gbit/s or higher, since a slight transition in the status of a transmission path changes the chromatic dispersion, it is forecast that a dynamic dispersion compensation is needed.
FIG. 17 is a diagram showing the structure of a prior art tunable dispersion compensation device as disclosed in Japanese patent application publications No. 10-221658, No. 2000-235170, and No. 2000-252920, to solve the above-mentioned problem. In the figure, reference numeral 2 denotes an optical waveguide in which a chirped grating having a grating pitch (i.e., grating period) that continuously changes along its optical axis is formed, reference numerals 3-1 to 3-n denote a plurality of heaters for producing a desired temperature distribution in the optical waveguide 2, respectively, and reference numerals 8-1 to 8-n denote a plurality of electrodes via each of which an electric current flows into a corresponding heater, respectively.
In operation, since the nearer to an input/output end of the optical waveguide 2 the longer grating pitch and hence the longer Bragg reflection wavelength the grating has, spectral components having longer wavelengths in an optical signal are reflected back at locations nearer to the input/output end of the optical waveguide 2 and are output via the input/output end. In other words, spectral components of shorter wavelengths in an optical signal reach locations within the optical waveguide 2, which are further from the input/output end of the optical waveguide 2, and are reflected back at the locations corresponding to the Bragg reflection wavelengths decided by the grating pitches. Therefore, different spectral components in an optical signal are reflected back at different locations in the optical waveguide 2 and thus have different delays. As a result, when an optical signal with a broadened pulse width in which spectral components of shorter wavelengths exist at more forward parts thereof is incident on the optical waveguide 2, the pulse width of the optical signal is compressed and is emitted out of the optical waveguide 2.
The optical waveguide 2 is made of a material, such as silica glass, whose refractive index changes according to its temperature. A desired temperature distribution can be produced along the length of the optical waveguide 2 by adjusting the electric power applied to each of the plurality of heaters 3-1 to 3-n by way of a corresponding one of the plurality of electrodes 8-1 to 8-n. When the optical waveguide 2 is heated by the plurality of heaters 3-1 to 3-n so as to have a desired temperature distribution, the grating pitch and refractive index of each segment of the chirped grating formed in the optical waveguide 2 which is heated by a corresponding one of the plurality of heaters change. As a result, the Bragg reflection wavelength of each segment of the chirped grating changes. The chromatic dispersion provided for an input optical signal by the optical waveguide 2 therefore changes.
Neither of the above-mentioned Japanese patent application publications discloses a concrete method of adjusting the electric power supplied to each of the plurality of heaters 3-1 to 3-n for the purpose of dynamic dispersion compensation. For example, a method of adjusting the electric power to be applied to each of the plurality of heaters by changing the resistance value of a resistor connected in series to a corresponding one of the plurality of heaters can be devised. In this case, a variable resistor is connected to each of the plurality of heaters, and the resistance value of the variable resistor is changed and the electric power supplied to each of the plurality of heaters is therefore adjusted according to a desired temperature distribution to be produced in the chirped grating.
FIG. 18 is a diagram showing the structure of a prior art tunable dispersion compensation device that can dynamically compensate for chromatic dispersion, as disclosed in Japanese patent application publication No. 2000-137197, and FIG. 19 is a diagram schematically showing the structure of an optical fiber communication system including the tunable dispersion compensation device 91 shown in FIG. 18, as disclosed in Japanese patent application publication No. 2000-244394. In FIG. 18, reference numeral 9 denotes a resistive thin film whose thickness changes linearly along the length of an optical waveguide 2, reference numerals 27a and 27b denote electrodes via which an electric current is supplied to the resistive thin film 9, and reference numeral 28 denotes a direct-current power supply for supplying the electric current to the resistive thin film 9 by way of the electrodes 27a and 27b. Furthermore, in FIG. 19, reference numeral 40 denotes an optical transmitter for multiplexing and transmitting a plurality of optical signals of different wavelengths each of which carries information, reference numeral 50 denotes an optical fiber transmission line via which the plurality of multiplexed optical signals are transmitted, reference numeral 90 denotes a dispersion compensation module provided with the tunable dispersion compensation device 91 shown in FIG. 18, on optical circulator 92 for guiding an optical signal which has propagated through the optical fiber transmission line 50 to the tunable dispersion compensation device 91, and a data integrity monitor 93 for monitoring the integrity of data transmitted on the system and for feeding the monitored data integrity back to the tunable dispersion compensation device 91, and reference numeral 100 denotes an optical receiver for receiving the multiplexed optical signals dispersion-compensated by the dispersion compensation module 90, and for demultiplexing the multiplexed optical signals into the plurality of optical signals so as to demodulate information which each of the plurality of optical signals carries.
In operation, the direct-current power supply 28 supplies an electric current to the resistive thin film 9 by way of the electrodes 27a and 27b. As a result, local resistive heating is generated along the length of the optical waveguide 2 so that it is proportional to the local resistance of the resistive thin film 9. This local heating generates a temperature gradient along the length of the grating formed in the optical waveguide 2 to cause the grating to produce a chirp. As previously mentioned, the resistive thin film 9 is so constructed that its resistance varies linearly along the length of the optical waveguide 2, and the grating can achieve a linear chirp.
The optical transmitter 40 multiplexes a plurality of optical signals of different wavelengths, each of which carries information, and then sends them out to the optical fiber transmission line 50. As previously mentioned, the optical fiber transmission line 50 can provide abnormal dispersion (negative group velocity dispersion) for an optical signal of a wavelength of 1550 nm, the chromatic dispersion being typically equal to about 17 ps/nm/km. In other words, spectral components of shorter wavelengths included in an optical signal propagate through the optical fiber transmission line 50 faster than other spectral components of longer wavelengths, and if the optical fiber transmission line 50 has a length of 50 km the accumulated chromatic dispersion can be about 850 ps/nm. The dispersion compensation module 90, which contains the tunable dispersion compensation device 91 shown in FIG. 19, can continuously adjust the chromatic dispersion within a range of xe2x88x92300 ps/nm to xe2x88x921350 ps/nm. The multiplexed optical signals dispersion-compensated by the dispersion compensation module 90 are further transmitted to the optical receiver 100. The optical receiver 100 demultiplexes the received, multiplexed optical signals into the plurality of optical signals so as to demodulate information which each of the plurality of optical signals carries.
A problem with a prior art tunable dispersion compensation device constructed as above is that since electric power is consumed in a resistor, which is connected in series to each of a plurality of heaters which heat a chirped grating, for adjusting the electric power supplied to each of the plurality of heaters, the electric power consumption in the entire system increases.
Although the other prior art tunable dispersion compensation devices as disclosed in Japanese patent application publication No. 10-221658 and so on do not implement a concrete method of producing a desired temperature distribution by adjusting the electric power to be applied to each of the plurality of heaters 3-1 to 3-n, how to actually control the temperature distribution of the chirped grating according to the chromatic dispersion in the optical fiber which can vary with time, and how to carry out the control with efficiency are important issues.
In addition, although the other prior art tunable dispersion compensation device as disclosed in Japanese patent application publication No. 2000-137197 produces a desired temperature gradient in the optical waveguide 2 by adjusting an electric current that flows through the resistive thin film 9, a problem with the prior art tunable dispersion compensation device is that when a change in the chromatic dispersion to be compensated for results from a change in the status of the optical fiber transmission line 50, it is difficult to change the temperature gradient while maintaining the center of a range of wavelengths over which the optical waveguide 2 can perform dispersion compensation. Another problem with the prior art tunable dispersion compensation device that employs the resistive thin film 9 is that it only generates a fixed temperature gradient for a constant voltage applied to the resistive thin film, and it is therefore difficult to produce a desired temperature gradient due to heat conduction and so on and it is difficult to deal with variations in the grating pitches which occur when the chirped grating is written into the optical waveguide 2. In addition, when the prior art optical fiber communication system that performs dispersion compensation by using the dispersion compensation module 90 which contains such a prior art tunable dispersion compensation device utilizes many channels over a wider range of optical wavelengths to transmit larger amounts of information, the wavelength dependency of the chromatic dispersion of the optical fiber transmission line 50 comes to the surface. When a group delay provided for a spectral component in an optical signal varies with its wavelength, it is necessary to produce a nonlinear temperature gradient in the optical waveguide 2. A problem with the prior art tunable dispersion compensation device disclosed in Japanese patent application publication No. 2000-137197 is, however, that it is difficult to produce a nonlinear temperature gradient with a high degree of accuracy, and it is difficult to achieve dynamic dispersion compensation over a wide range of wavelengths.
Furthermore, another problem with the prior art optical fiber communication system constructed as above is that since each of the plurality of optical signals demultiplexed has residual chromatic dispersion which remains to be compensated for and chromatic dispersion due to nonlinear effects of the optical receiver, and the chromatic dispersion varies with time, it is difficult to ensure complete restoration of all the plurality of optical signals received at the optical receiver and at times.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a tunable dispersion compensation device that can dynamically compensate for chromatic dispersion in an optical fiber transmission line by producing a temperature distribution (i.e., temperature gradient) in a chirped grating with efficiency and changing the temperature distribution according to the chromatic dispersion which varies with time, and an optical receiver provided with the tunable dispersion compensation device.
It is a further object of the present invention to provide an optical fiber communication system that can efficiently, precisely, and dynamically compensate for chromatic dispersion in the system including an optical fiber transmission line whose chromatic dispersion varies with time and an optical receiver.
In accordance with an aspect of the present invention, there is provided a tunable dispersion compensation device comprising: an optical waveguide having a grating; a plurality of heaters arranged along an optical axis of the optical waveguide; and a pulsed-current supplying unit for producing a desired temperature distribution in the grating by supplying a plurality of pulsed currents to the plurality of heaters, respectively. Accordingly, the present invention offers an advantage of being able to perform dispersion compensation with efficiency.
In accordance with another aspect of the present invention, the grating is a chirped grating.
In accordance with a further aspect of the present invention, the pulsed-current supplying unit includes a pulse width control unit for adjusting pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively, according to the desired temperature distribution to be produced in the grating.
In accordance with another aspect of the present invention, the pulsed-current supplying unit supplies the plurality of pulsed currents to the plurality of heaters at different times, respectively. Accordingly, the tunable dispersion compensation device can reduce the peak value of an electric current that flows from the pulsed-current supplying unit to the plurality of heaters.
In accordance with a further aspect of the present invention, the pulsed-current supplying unit divides the plurality of pulsed currents into a plurality of groups and supplies pulsed currents included in different groups to corresponding heaters at different times, respectively.
In accordance with another aspect of the present invention, the pulsed-current supplying unit includes a DC power supply, an EMI elimination filter for eliminating noise included in a DC output from the DC power supply, and a switching unit for generating the plurality of pulsed currents from an output of the EMI elimination filter. Accordingly, the present invention offers an advantage of being able to reduce the load imposed on the DC power supply which is the source of the pulsed-current supplying unit and to downsize the EMI elimination filter which is used to eliminate switching noise included in the output of the DC power supply.
In accordance with a further aspect of the present invention, the pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively are increased or decreased in order that the plurality of heaters respectively associated with the plurality of pulsed currents are arranged along the optical axis of the waveguide.
In accordance with another aspect of the present invention, the pulse widths of the plurality of pulsed currents are increased or decreased linearly.
In accordance with a further aspect of the present invention, the pulse width control unit includes a pulse width determining unit for determining the pulse widths of the plurality of pulsed currents based on an initial value and a pulse width increment.
In accordance with another aspect of the present invention, the pulse width control unit includes a correction unit for correcting the pulse widths of the plurality of pulsed currents determined by the pulse width determining unit using a plurality of correction coefficients that are predetermined for the plurality of heaters, respectively.
In accordance with a further aspect of the present invention, the pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively, are increased or decreased nonlinearly.
In accordance with another aspect of the present invention, the pulse width control unit includes a pulse width determining unit for determining the pulse widths of the plurality of pulsed currents based on an initial value, a pulse width increment, and a plurality of correction coefficients that are predetermined for the plurality of heaters, respectively.
In accordance with a further aspect of the present invention, the pulsed-current supplying unit includes a DC power supply, a switching unit including a plurality of switches (referred to as first to nth switches from here on) each for generating a pulsed current from a DC output from the DC power supply in response to a control pulse applied thereto, and a control pulse generation unit for generating a control pulse to be supplied to the (i+1)th (i=1 to nxe2x88x921) switch based on the pulsed current generated by the ith switch. Accordingly, the present invention offers an advantage of being able to prevent two adjacent heaters from being turned on at the same time and to ensure that a plurality of pulsed currents reach the plurality of heaters at different times, respectively. In addition, the present invention offers another advantage of being able to further reduce the load imposed on the DC power supply which is the source of the pulsed-current supplying unit and to downsize the EMI elimination filter which is used to eliminate switching noise included in the output of the DC power supply.
In accordance with another aspect of the present invention, the pulsed-current supplying unit includes a DC power supply, a switching unit including a plurality of switches (referred to as first to nth switches from here on) each for generating a pulsed current from a DC output from the DC power supply in response to a control pulse applied thereto, and a control pulse generation unit for generating a control pulse to be supplied to the (i+1)th (i=1 to nxe2x88x921) switch based on a pulse which is delayed by a predetermined time interval with respect to a control pulse supplied to the ith switch.
In accordance with a further aspect of the present invention, there is provided an optical receiver comprising: a dispersion detector for detecting chromatic dispersion of an optical signal incident thereon, and for generating a control signal having a value corresponding to the detected chromatic dispersion; a tunable dispersion compensation device including an optical waveguide having a grating, a plurality of heaters arranged along an optical axis of the optical waveguide, and a pulsed-current supplying unit for producing a desired temperature distribution in the grating by supplying a plurality of pulsed currents to the plurality of heaters, respectively, according to the control signal from the dispersion detector; and an optical circulator for guiding the optical signal with chromatic dispersion to be compensated for to the dispersion compensation device, and for guiding the optical signal compensated by the dispersion compensation device to the dispersion detector. Accordingly, the tunable dispersion compensation device can quickly change the temperature distribution of the optical waveguide by changing the pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively, according to the chromatic dispersion detected by the dispersion detector, and therefore the optical receiver can achieve quick-response dynamic dispersion compensation.
In accordance with another aspect of the present invention, the pulsed-current supplying unit of the tunable dispersion compensation device includes a pulse width control unit for adjusting pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively, according to the desired temperature distribution to be produced in the grating.
In accordance with a further aspect of the present invention, the pulsed-current supplying unit of the tunable dispersion compensation device supplies the plurality of pulsed currents to the plurality of heaters at different times, respectively.
In accordance with another aspect of the present invention, there is provided an optical fiber communication system including an optical transmitter for multiplexing a plurality of optical signals having different wavelengths, an optical fiber transmission line via which the plurality of optical signals multiplexed by the optical transmitter are transmitted, an optical receiver for demultiplexing the plurality of optical signals multiplexed and received via the optical fiber transmission line and for demodulating information that the plurality of optical signals carry, and a tunable dispersion compensation unit for compensating for chromatic dispersion of each of the plurality of the optical signals transmitted via the optical fiber transmission line, the optical tunable dispersion compensation unit comprising: at least a tunable dispersion compensation device including an optical waveguide having a grating, a plurality of heaters arranged along an optical axis of the optical waveguide, and a pulsed-current supplying unit for producing a desired temperature distribution in the grating by supplying a plurality of pulsed currents to the plurality of heaters, respectively. Accordingly, the present invention offers an advantage of being able to efficiently, precisely, and dynamically compensate for residual chromatic dispersion in the optical fiber communication system and chromatic dispersion in the optical receiver.
In accordance with a further aspect of the present invention, the optical fiber communication system further comprises a static dispersion compensation unit coupled to the optical fiber transmission line, for compensating for a different, fixed amount of chromatic dispersion of each of the plurality of optical signals transmitted via the optical fiber transmission line.
In accordance with another aspect of the present invention, the optical tunable dispersion compensation unit includes a plurality of optical receiving unit disposed in the optical receiver, each for compensating for chromatic dispersion of a corresponding one of the plurality of optical signals demultiplexed, and each of the plurality of optical receiving unit comprises a dispersion detector for detecting chromatic dispersion of a corresponding one of the plurality of optical signals, and for generating a control signal having a value corresponding to the detected chromatic dispersion, a tunable dispersion compensation device including an optical waveguide having a grating, a plurality of heaters arranged along an optical axis of the optical waveguide, and a pulsed-current supplying unit for producing a desired temperature distribution in the grating by supplying a plurality of pulsed currents to the plurality of heaters, respectively, according to the control signal from the dispersion detector, and an optical circulator for guiding the optical signal with chromatic dispersion to be compensated for to the tunable dispersion compensation device, and for guiding the optical signal compensated by the tunable dispersion compensation device to the dispersion detector.
In accordance with a further aspect of the present invention, the pulsed-current supplying unit of the tunable dispersion compensation device has a pulse width control unit for adjusting pulse widths of the plurality of pulsed currents supplied to the plurality of heaters, respectively, according to the desired temperature distribution to be produced in the grating.
In accordance with another aspect of the present invention, the pulsed-current supplying unit supplies the plurality of pulsed currents to the plurality of heaters at different times, respectively.
In accordance with a further aspect of the present invention, there is provided a method of compensating for chromatic dispersion of an optical signal by using an optical waveguide having a grating, the method comprising the step of: producing a desired temperature distribution in the grating by supplying a plurality of pulsed currents to a plurality of heaters, respectively, the plurality of heaters being arranged along an optical axis of the optical waveguide.
In accordance with another aspect of the present invention, the method further comprises the steps of: detecting chromatic dispersion of the optical signal, generating a control signal having a value corresponding to the detected chromatic dispersion, and supplying a plurality of pulsed currents to the plurality of heaters, respectively, according to the control signal.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.